Managers believe the extra precautions will keep Atlantis and its seven-person crew as safe as possible if, indeed, the shuttle lifts off with a European lab intended for the international space station.

On Saturday, two engineering departments at NASA recommended delaying the launch and doing additional testing to figure out why so many fuel gauges acted up during Thursday's launch attempt. But in the end, they did not oppose trying for a liftoff, said LeRoy Cain, chairman of the mission management team.

"We'll fill up the tank and we'll see what we get," Cain said. "If we meet our criteria, we'll go fly and if we don't, we'll scrub and we'll get a good tanking test and we'll go forward from there."

Under the new rules, NASA will proceed with the countdown only if all four of the gauges in Atlantis' big hydrogen tank are working properly. Two of them failed when the shuttle's tank was filled for liftoff on Thursday and a third one subsequently acted up.

NASA passed up launch tries on Friday and Saturday because of the perplexing problem, which has plagued the shuttle program off and on for more than two years. After meeting again Saturday, shuttle managers decided to press ahead with a Sunday afternoon liftoff, but only if all the fuel gauges behave.

The fuel gauges - officially known as engine cutoff sensors - are part of a critical backup system for preventing the shuttle's main engines from running too long during the climb to orbit. If the engines kept running and the fuel tank was empty because of a leak or other unexpected trouble, they could ignite or explode.

Last year, after struggling in vain with the problem, NASA loosened its launch rules to require only three of the four fuel gauges to be working before liftoff. Managers went back to the four-of-four rule Saturday - for this mission only - after concluding that Atlantis' system was suspect and that it would be too risky to attempt a launch without every single gauge functioning.

NASA also decided to shorten the five-minute launch window to a single minute to put Atlantis on a more direct path to the space station and keep more fuel in the tank, just in case of sensor trouble. And new instrumentation for monitoring the condition of the fuel gauges will be used during flight, enabling flight controllers and the astronauts to intercede in case of multiple failures.

The plan was put forth by the astronauts themselves.

Cain said the rules put an added burden on commander Stephen Frick and his crew, as well as Mission Control. He said Frick was "very deeply involved" in all of the decisions.

Packed aboard Atlantis is the European Space Agency's $2 billion (EUR 1.4 billion) science laboratory, Columbus. It will be the second lab added to the space station; Japan's Kibo lab, or Hope, will follow on successive shuttle flights next year.

Columbus has been waiting to fly for years. It was stalled first by NASA space station design problems, then by Russian space station holdups, then by the Columbia tragedy in 2003, which grounded all shuttle flights for 2Ѕ years.

NASA is up against a tight deadline for launching Atlantis in December. If the shuttle is not flying by Thursday or possibly Friday, the mission will have to wait until January because of unfavorable sun angles and computer concerns.